The operation of an internal combustion engine such as, for example, a diesel, gasoline, or natural gas engine, may cause the generation of undesirable emissions. These emissions, which may include particulates and oxides of nitrogen (NOx), are generated when fuel is combusted in a combustion chamber of the engine. An exhaust stroke of an engine piston forces exhaust gas, which may include these emissions, from the engine. If no emission reduction measures are in place, these undesirable emissions will eventually be exhausted to the environment.
Research is currently being directed towards decreasing the amount of undesirable emissions that are exhausted to the environment during the operation of an engine. It is expected that improved engine design and improved control over engine operation may lead to a reduction in the generation of undesirable emissions. Many different approaches such as, for example, exhaust after-treatments, have been found to reduce the amount of emissions generated during the operation of an engine. The use of exhaust aftertreatments requires periodic regeneration of the aftertreatment systems. Aftertreatment regeneration may require incremental fueling in the form of late post injections, which require additional capacity of the fuel delivery system.
Another method of reducing undesirable emissions involves improving the combustion characteristics of the engine. This may be accomplished, for example, by implementing homogeneous charge compression ignition (HCCI) in the engine. In a HCCI engine, fuel enters the engine with the intake air prior to or at the start of the compression stroke and both the intake air and fuel are heated in the compression stroke. HCCI operation requires precise control over fuel flow and fuel delivery pressure. For example, HCCI may require early fuel injection timing and a reduced fuel injection pressure that limits spray penetration during the associated relatively low cylinder air densities.
The fuel flows and pressures required to efficiently run low emission engines utilizing after-treatment solutions and/or HCCI may vary greatly. In order to ensure that a sufficient supply of fuel at a sufficient pressure is always available to fulfill the demands of the engine, the fuel delivery system should be capable of supplying the largest anticipated quantity of fuel at the highest anticipated pressure that could be demanded. Unfortunately, this design approach typically results in a fuel system that is over designed for typical engine operation and may, therefore, be inefficient for the majority of the engine's operation.
As described in U.S. Pat. No. 6,253,735 to Miyajima et al., issued on Jul. 3, 2001, a fuel system may include multiple, selectively actuated fuel pumps that are operable to deliver an increased fuel flow rate when demanded. When the demand for fuel is low, one of the fuel pumps may be deactivated, thereby reducing the load on the engine and increasing engine efficiency. However, the fuel system described in the '735 patent to Miyajima et al., which includes multiple pumps, regulators, valving arrangements, and controllers, may be overly complex and costly.
The fuel pumping system of the present invention solves one or more of the problems set forth above.